Johann Michael Haydn

Haydn's sacred choral works are generally regarded as being his most important, including the Requiem pro defuncto Archiepiscopo Sigismundo (Requiem for the death of Archbishop Siegmund) in C minor, which greatly influenced the Requiem by Mozart, Missa Hispanica (which he exchanged for his diploma at Stockholm), a Mass in D minor, a Lauda Sion, and a set of graduals, forty-two of which are reprinted in Anton Diabelli's Ecclesiasticon. He was also a prolific composer of secular music, including thirty symphonies and partitas, a number of concerti and chamber music including a string quintet in C major which was once thought to have been by his brother Joseph.

The confusion continues to this day: often the Classical Archives page for Joseph Haydn has some MIDI files of Michael Haydn compositions, which are eventually moved to the general H page.

Michael Haydn was the victim of another case of posthumous mistaken identity: for many years, the piece which is now known as Michael Haydn's Symphony No. 25 was thought to be Mozart's Symphony No. 37 and assigned K. 444. The confusion arose because an autograph was discovered which had the opening movement of the symphony in Mozart's hand, and the rest in somebody else's. It is now thought that Mozart had composed a new slow introduction for reasons unknown, but the rest of the work is known to be by Michael Haydn. The piece, which had been quite widely performed as a Mozart symphony, has been performed considerably less often since this discovery in 1907.

Michael Haydn never compiled a thematic catalog of his works, nor did he ever supervise the making of one. The earliest catalog was compiled in 1808 by Nikolaus Lang for 'Biographische Skizze'. In 1907 Lothar Perger compiled a catalogue of his orchestral works for 'Denkmäler der Tonkunst in Österreich', which is much more reliable. And in 1915 Anton Maria Klafsky undertook a similar work regarding the sacred vocal music.

Some of Haydn's works are referred to by Perger numbers, from the thematic catalog of his works compiled by Lothar Perger in 1907.